Haematoporphyrin derivatives of uncertain specific composition (HpD) have been used in cancer treatment, having been found to locate in tumours and other tissues after injection into the bloodstream, and to sensitise cells to light irradiation (transport in the blood is believed to be largely in association with the serum albumin). Irradiation, with a laser or other source, may be direct or indirect, for example using fibre optics. Irradiated cells, unless deeply pigmented, are rapidly killed to a depth depending on the light penetration. The mechanism of cell killing in photo-therapy is believed to be largely by production of singlet oxygen. This is produced by transfer of energy from the light-excited porphyrin molecule to an oxygen molecule. Singlet oxygen is highly reactive. It is believed to oxidise cell membranes so that they are damaged and become incapable of exerting their function of controlling the cell's internal operation. This rapidly leads to cell death.
Selectivity of damage for various tissues, including tumours, is governed by a variety of factors, including selective uptake, selective vulnerability and selective ability to repair damage (cf. Albert A. A. "Selective Toxicity", fifth edition, Chapman & Hall, London 1973). In tumours an additional factor is the precarious nature of tumour blood supply and its particular vulnerability to photo-dynamic damage as compared with the robust and well-established blood supply of normal tissues. All the above factors vary from one tissue to another and from one compound to another, so that the pattern of tumour and normal tissue damage may be expected to vary.
In EP-A-0,186,962, the present applicants made proposals to improve on HpD by finding well characterised and thus more exactly controllable compounds. Other aims were to find compounds activated by light at wavelengths longer than those used to activate HpD so as to exploit deeper penetration of longer wavelength radiation, and to increase effectiveness generally, as in many anatomical sites, such as the brain, HpD has been found to sensitise normal cells unduly as well as cancerous cells. The wavelength at which a photo-sensitising compound is activated is one factor in its in vivo effectiveness. Other things being equal, the longer the activating wavelength within the visible range, the greater the tissue penetration of light and therefore the greater the depth of damage. Thus, compounds activated at 650-660 nm might be expected to produce greater depths of damage than HpD, which is activated at 625-630 nm. Successful photo-therapy depends on the ability to produce severe tumour damage without unacceptable damage to contiguous normal tissues. The compounds used according to EP-A-0,186,962 are porphyrins tetra-meso-substituted by aromatic groups bearing hydroxy, amino or sulphydryl groups, some new as compounds others old.
The present invention also concerns medicaments for therapy of tumours susceptible to necrosis when an appropriate compound is administered to locate in the tumour followed by illumination of the tumour with light of a wavelength absorbed by the compound, but the compounds are new and the invention therefore lies primarily in them. They are dihydro porphyrins (chlorins) I, and corresponding tetra-hydro porphyrins II and III, of the formulae: ##STR2## wherein each Ar, the same or different, is an aromatic group with one or more hydroxy (--OH) substituent groups. For example, and showing the ring numbering, the dihydro porphyrins (and the tetra-hydro porphyrins correspondingly) may be: ##STR3## wherein each R (one or more in each ring n=1 to 3) is an ortho, meta or para positioned hydroxy substituent group, particularly to give polyhydroxyphenyl compounds. Said substituent groups may be in the same or different positions on their respective aromatic groups. Any said substituent group may be free or itself substituted, for example by alkyl or acyl groups preferably C.sub.1 to C.sub.4 and the compounds when in such form are within the claims herein. The nucleus or the substituent rings may be substituted further, provided pharmacological tolerability, appreciable solubility in water (required so that the drug may be adminstered intravenously to ensure rapid distribution to the tumour), absorption of light at the red end of the spectrum, and take up in cancerous tissue are retained, and again the compounds when in such form are to be understood as within the claims herein.
Any of the compounds further may be in the form of derivatives such as salts at acidic or basic centres, metal complexes (e.g. Zn, Ga), or hydrates or other solvates particularly with lower, e.g. C.sub.1 -C.sub.4 aliphatic alcohols and again such derivatives are within the claims.
It is preferred that one or more of the said substituent groups should be of a kind and in a form able to ionise at physiological pH, to increase the absorption in the red part of the spectrum, that is in the portion that most effectively penetrates tissue. Compounds not very soluble in themselves may be solubilised by the presence of suitable groups such as sulphonate groups.
The invention further extends to the medicaments, to their preparation and to the method of therapy itself.
Various specific compounds within the claims are given in Table 1 below:
TABLE 1 ______________________________________ CODE NAME (DI- NATURE OF "Ar" IN HYDRO or TETRA- NUMBER FORMULA I or II HYDRO FORM OF) ______________________________________ ##STR4## HK7 2 ##STR5## acetyl HK7 3 ##STR6## ortho-HK7 4 ##STR7## ortho-HK7 methyl ether 5 ##STR8## meta-HK7 6 ##STR9## acetyl meta-HK7 ______________________________________
The invention is illustrated in the following Examples, in which the compounds are made by the reduction of the corresponding porphyrins, without protection of the latter. The general method of reduction is not new (Whitlock et al., J. Amer. Chem. Soc., 1969, 91, 7485) but the products are new. It is unexpected that they can be obtained without prior protection of phenolic hydroxy functions, especially when an oxidative step is required in the work up. When required, a chloranil dehydrogenation step is included to oxidise tetra-hydro porphyrins formed back to the dihydro form.